Recent numbers
of the Bulletin of the Association of Field Engineers have made
it clear that acoustic determination of depth and position have
become an essential part of the Bureau's work. The development
work is now well organized and a considerable part of the personnel
has knowledge of the details. The success of this work warrants
the placing on record of the history of its inauguration, when
none of these favorable conditions existed.

Shore
Radio Acoustic Ranging Station on San Clemente Island,
California, 1925. Radio acoustic ranging operations on
the GUIDE.

In
the early part of 1923 successful use of acoustic methods in
obtaining deep soundings through the use of the sonic depth
finder by the Navy Department, as well as similar work by the
French and British, indicated that these methods might be adapted
to our special problems. The British had also been successful
in locating a vessel by radio acoustic methods though without
the automatic return to the vessel of the radio signal which
has made our work so effective. Colonel E. Lester Jones, then
Director, with the advice of the officers concerned decided
that the Coast and Geodetic Survey should take up this work.
It was felt that the difficulties of location of position off
the Pacific Coast on account of fog made it desirable that both
depth and position determination should be developed at the
same time.

My
personal association and relation to the work which occasions
the writing of this article should be explained, especially
since at that time I was Chief of the Division of Terrestrial
Magnetism, and continued that duty during my association with
the acoustic work. There were no personnel familiar with the
details of this work, but I was brought into close contact with
several phases of it at New London, Connecticut, and London,
England, while in the Navy during the World War. I was also
personally acquainted with those who had done the development
work and their method of attacking the problems. I was therefore
sufficiently familiar with the general problem, both from the
viewpoint of the fundamental research and development and from
the viewpoint of hydrographic work, to be qualified to serve
as a kind of liaison officer between those doing the development
work and the as yet untrained Coast Survey personnel.

It
was obvious to me that the radio acoustic work could not be
carried on along previous lines, that is, by recording at several
stations and then sending the distance by radio from each station.
Tests made at the Washington office under my direction indicated
that recording aboard ship with the necessary accuracy was possible
and the problem was outlined with this feature and the automatic
sending of radio signals by the arriving sound wave. The Bureau
of Standards was then consulted, funds were transferred, and
the development work was assigned to Dr. E. A. Eckhardt who
had previously developed radio longitude apparatus for the Bureau.
His principal assistant on the work was M. Keiser.

The
ship GUIDE, which had recently been put into commission, based
at New London during the preliminary experimental work. During
the development period R. F. Luce was in command, K. T. Adams,
executive officer, and J. H. Service, whose previous advanced
work in physics proved invaluable, was also assigned to the
vessel. In the radio acoustic experiments off New London, Colonel
R. S. Abernethy and Major H. C. Allen, U. S. Coast Artillery
Corps, gave valuable advice and assistance.

The preliminary tests made steady progress, though beset with
many difficulties, and were completed in late November. During
this period there were several cruises to obtain practice in
the use of the sonic depth finder. On one occasion a test was
made to find out how far the bomb signals would carry. It was
determined that a signal could be transmitted and received accurately
at a distance of 55 miles, with an average depth of 20 fathoms.
This success proved somewhat misleading, since later attempts
to use radio acoustic methods on the Atlantic Coast proved that
such results are possible only under exceptionally favorable
conditions. Just before the GUIDE started for the Pacific Coast,
a demonstration cruise was made to prove to Colonel Jones and
a party of Coast Survey Officers and guests, among whom was
Captain Bob Bartlett, that the apparatus had passed the preliminary
tests and was ready for test in actual surveying.

There were a number of difficulties and annoyances having chiefly
to do with mutual interference in radio transmission in a region
where there were many sources of interference. On one occasion
a test was started during the broadcasting of a world series
game and we were promptly invited to postpone operations. It
should be understood that control of wavelengths was not then
as rigid as now, and the short waves were not available so that
we were not far from the broadcast and marine bands. On one
occasion Mr. Keiser informed the listening nautical world that
the ship would "park" in a certain harbor at 5:00 P.M.

The GUIDE sailed in late November for her field of duty near
San Diego, California, and proceeded via Porto Rico (sic) to
the Panama Canal in order to secure a wide range of depths for
testing the sonic depth finder. In the Pacific she followed
a prescribed course with relation to previous soundings by vessels
of the Bureau passing between the east and west coasts. This
particular course followed a series of deep troughs off the
Central American and Mexican coasts, a part of which was previously
known as the Acapulco Deep, though depths much greater than
any previously charted were found. The trough was found to have
a maximum depth at least twice as great as the depths on either
side. This was later found to be an active earthquake region.

The program of the GUIDE included the taking of acoustic soundings
at the same time as the wire soundings. Temperatures were observed
and water samples obtained so that shortly after the end of
the cruise salinity determinations had been made by the Scripps
Institution at La Jolla, California. The skill of Commander
Luce and his complement was evidenced by the fact that all wire
soundings, except those in regions of very strong currents such
as the Gulf Stream and in a few places in the Pacific, were
vertical and that while soundings were taken in depths up to
4,600 fathoms (Nares Deep) in the Atlantic and up to 3,500 fathoms
in the Pacific, no sounding wire or attached apparatus was lost.
Such a record with piano wire is probably without precedent.
On one occasion in the Pacific, with specially favorable weather
conditions, and a depth of about two thousand fathoms, three
separate sets of thermometers and water specimen cups were attached
at different depths and all were recovered.

As a result of the route selected, the range of depths was
exceptionally great and the problem immediately developed as
to what velocity of sound to use. It was evident that any attempt
to use the same value for all soundings, as in previous practice,
gave results considerably at variance with the simultaneous
wire soundings. This constituted a problem better suited to
an office force than to a ship personnel actively engaged in
surveying operations, but since it had to be solved, Mr. Service
and I gave particular attention to its solution. It could be
accepted that the velocity varied with temperature, salinity
and pressure, but we had available none of the fundamental data.
The first attempt was made possible through Mr. Service's discovery
of the volumes of the results of the CHALLENGER expedition,
at the University of Porto Rico at Rio Piedras, near San Juan.
He copied enough data to show that we were on the right track.
however, the method was very cumbersome and not practical.

On arrival at San Diego consultation with Dr. Geo. F. McEwen
of the Scripps Institution resulted in our securing tables of
constants of sea water prepared by V. Bjerknes under the auspices
of the Carnegie Institution of Washington. We were able to develop
comparatively simple formulas for obtaining the velocity and,
by temporarily ignoring the adiabatic correction, we were able
to compute satisfactory values of the velocity. Later these
results were expanded into Special Publication No. 108, "Velocity
of Sound in Sea Water." We adopted the plan of considering the
water in layers and working out the mean velocity for each,
not rigidly correct but with no important error except in the
upper layer. In all the deeper sounding the effect of error
in the upper layer was not serious. The values in Special Publication
No. 108 do not differ materially from those given in British
Admiralty Tables H D 282, although the latter are better suited
for shoal water work and for computing horizontal velocities.
With regard to the cruise of the GUIDE, the comparison of the
wire and acoustic soundings with velocity of sound values computed
as has been described showed an average agreement within one
per cent, all that could be expected.

On a special survey off the California coast, a special effort
was made to obtain the highest accuracy in the acoustic determinations
and in the soundings at depths of 600 to 800 fathoms, and it
was demonstrated that the acoustic method had the possibilities
required in shoal water work. However, it was found that the
sonic depth finder as designed at the time was not suited to
shoal water. It should be recognized that it was work of this
type which laid the foundation for the later successful use
of the fathometer.

On arrival at San Diego work started on the radio acoustic
ranging apparatus and installations were made at Oceanside and
at La Jolla, the latter stations being peculiarly suited to
development of the work through the courtesies afforded by Dr.
T. Wayland Vaughan, Director of the Scripps Institution, on
whose pier the station was placed. Valuable experience was obtained,
numerous difficulties were solved and the technique was gradually
worked out. The surveying work was continuous, but only a portion
of the position location was by radio acoustic ranging. Great
difficulty was found in securing proper detonation of the bombs,
and for this reason some studies of sound transmission which
were intended had to be deferred to a later date. An interesting
use was made of one of the shore stations. The inshore work
of the ship had caught up with the triangulation so that on
one afternoon, the results of the day's work of the shore triangulation
party was sent off by radio telephone, computed, and used the
next day by the ship.

By April, after a visit of inspection by Captain W. E. Parker,
Chief of the Division of Hydrography and Topography, it was
agreed that the preliminary stages were over and that there
were no fundamental difficulties in the way of successful use
and that the foundation had been laid. I therefore returned
to my duties in Washington, thereby completing an experience
of unusual interest.

The complement of the GUIDE demonstrated that the transition
to the new form of hydrographic work could be made without fundamental
changes in the personnel, though the need for development work
by specialists had become evident. I find it difficult to single
out the work of individuals since so many contributed. The work
of chief radio operator Vincent of the GUIDE was specially noteworthy
because of his ability to solve radio problems as they arose,
often with limited facilities for the work. On one occasion
off the coast of Mexico part of the apparatus on the control
panel burned out and the oscillator (Fessenden) could be operated
only at full power, making soundings difficult and painful to
the operator. He made temporary repairs which lasted until arrival
at San Diego. H. E. McComb, magnetic observer, in addition to
making a magnetic survey between the Mexican border and Los
Angeles, gave advice, based on his experience with precise physical
instruments, to those in charge of shore stations, which helped
in establishing a proper technique.

There was one great surprise in the results. All of those who
discussed the project in its early stages questioned whether
radio acoustic work would be successful on the northwest Pacific
Coast of the United States on account of heavy surf noise interfering
with the signals and the difficulties of installing shore stations
and cables, while it was taken for granted that no difficulty
would be encountered on the Atlantic Coast. The exact opposite
proved the case and it is only recently in the course of the
Georges Bank work that use under Atlantic Coast conditions has
proven practicable.

In view of the fact that radio acoustic ranging is now used
so extensively, it may not be improper to point out that the
original conception of the problems was correct, and that while
scarcely a trace of the original apparatus remains in the present
equipment, there have been no fundamental changes in principle
and method.